LEAPing Over Wireless LANs

Dependable authentication is crucial on a wireless LAN. Understand the vulnerabilities of standard 802.11 authentication and consider the use of Cisco's LEAP as a security solution (and how it stacks up against new security like WPA).

Mutual authentication is a critical component of any network, especially a
wireless LAN. The user should authenticate with the network, and the edge device,
such as an access point or wireless router, should also authenticate with the
user. This establishes trust among everyone and avoids the perils of man-in-the-middle
attacks.

The original 802.11 standard, however, doesn't specify a robust form of authentication.
As a result, the designer of a wireless LAN must consider other methods of authentication.
In this tutorial, we'll focus on the Cisco proprietary Lightweight Extensible Authentication
Protocol (LEAP).

802.11 Authentication Vulnerabilities

When a wireless client becomes active, it searches the medium for beacons that
the access points broadcast. By default, the access point broadcasts beacons
containing the service set identifier
(SSID) of the access point, as well as other parameters. The access point only
enables association if the SSID of the client matches the SSID of the access
point. This process offers a basic but very weak form of authentication.

Of course the major vulnerability is the fact that the SSID is sent in the
clear (unencrypted), which makes it visible to wireless packet sniffers (e.g.,
Airmagnet and Airopeek). Because of this, a hacker can
easily identify the SSID within the beacon frame and authenticate with the wireless
LAN. Even if the access point is set to not broadcast the SSID (a feature available
in only a few access points), sniffers can still obtain the SSID from association
request frames sent from clients to the access point.

802.11 offers by default a form of authentication called open systems authentication.In this mode, the access point grants approval for any request for authentication.
The client simply sends an authentication request frame, and the access point
responds with an authentication approval. This lets anyone having the correct
SSID to associate with the access point.

The 802.11 standard also includes an optional, more advanced form of authentication
referred to as shared key authentication. This is a four step process. The client
sends an authentication request frame, and the access point responds with a
frame containing a string of characters called challenge text.

The client then encrypts the challenge text using the common wired
equivalent privacy (WEP) encryption key. The client sends the encrypted
challenge text back to the access point, which decrypts the text using the common
key and compares the result with the text originally sent. If the decrypted
text matches, then the access point authenticates the client.

This seems adequate for authentication, but a problem is that shared key authentication
only proves that the client has the correct WEP key. The weaknesses of WEP make
Man-in-the-Middle attacks possible. Through this type of attack, a hacker can
capture the original challenge packet as well as the WEP encrypted challenge
packet. From these two packets and the use of commonly available tools, a hacker
can easily mimic an authorized using and authenticate with the access point.

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As a result, standard 802.11 authentication mechanisms don't provide adequate
levels of protection. Of course another issue is that 802.11 authentication
mechanisms do not authentication the access point to the client.

Mutual Authentication. In wired networks, there is a direct physical
connection between the client and device, therefore the client is relatively
sure that it is communicating with the right network. That is not the case,
however, in a wireless LAN. Because there is no physical connection between
the two, the client must authenticate the network and the network needs to
authenticate the client, hence mutual authentication.

User-Based Authentication. Traditional802.11 authentication
only verifies radio devices, not actual users. Because of this, it is very
easy for an unauthorized user to access the network through a preauthorized
piece of equipment, for instance an employee's laptop. LEAP eliminates this
by authenticating the user through usernames and passwords, rather than just
the device.

Dynamic WEP Keys. Cisco LEAP uses 802.1X to continually
generate unique WEP keys for each user. Every 802.1X session timeout forces
clients to re-associate to the network, which is when the new WEP keys are
generated. The re-associations are not noticeable by users and are very important
to keep all sensitive data constantly encrypted.

Cisco is freely licensing its wireless LAN security suite, which includes LEAP,
to many chipset and radio card manufacturers. Many feel this move might undercut
the work of the 802.11i working group efforts. Because not all 802.11 products
support LEAP, though, you run into interoperability issues in a mixed vendor
environment (e.g., public wireless
LANs).

If you are sure that all products on the network will support LEAP -- for example,
a Cisco-only network -- LEAP is a viable security measure. In cases where there
are multiple client device vendors, consider another form of authentication.

What are the Alternatives?

Similar to LEAP, Wi-Fi Protected
Access (WPA) also securely authenticates wireless users to the network.
There are a couple of differences between the two, however. WPA specifies Temporal
Key Integrity Protocol (TKIP) for distributing dynamic encryption keys and then
lets the client use the EAP type of their choice. In WPA these two functions
are separate from each other.

Cisco uses a proprietary version of TKIP that is not interoperable with WPA.
Cisco is addressing the problem of interoperability with
WPA, however. An updated version will provide support for WPA.

Because of the common implementation of WPA in nearly all Wi-Fi products, WPA
is a better choice in most situations having a mixed vendor environment. In
addition, WPA is actually a subset of 802.11i, which makes WPA somewhat of a
better long term, interoperable solution.